Red Hat Enterprise Linux 7.5 -- Xt error: Can't open display: DISPLAY is not set

Problem

When starting an X11 application on RHEL 7.5 you get errors similar to below:

$ xterm

xterm: Xt error: Can't open display:

xterm: DISPLAY is not set

You logged into the RHEL host using PuTTY from a windows host. X Server is properly configured and running on the windows host.

Solution

Disable IPv6 on the RHEL host and restart the PuTTY session:

# sysctl -w net.ipv6.conf.all.disable_ipv6=0